We propose to evaluate the chemoprevention of prostate cancer among men who are, by virtue of having been diagnosed with High Grade Prostatic Intraepithelial Neoplasia (HGPIN), at a greatly elevated prostate cancer risk. After a biopsy-based diagnosis of HGPIN, each subject will be re- biopsied to minimize the probability he has PC missed by the first biopsy. Subjects will then be randomized to either 200 mcg/day of selenium in the form of L-selenomethionine or to a matching placebo. After randomization, each subjective will be followed for three years. The primary endpoint of this study is PC. Secondary endpoints will include two established Surrogate Endpoint Biomarkers (SEBs): Ki-67 evaluation of cell proliferation and TUNEL evaluation of apoptosis. Each of these endpoints will be evaluated for reproducibility, predictive association with subsequent PC, and for response to selenium supplementation. In addition, we propose a developmental investigation of two potential SEBs that are based on machine vision histometry: erosion of the basal cells of prostatic ducts and histometric characteristics of chromatin patterns. These developmental SEBs will be investigated for reproducibility.